Second screen below WebviewScaffold

Using WebviewScaffold with appBar and bottomNavigationBar bugs the Navigation.push.

I'm using the WebviewScaffold as my primary route "/", with two IconButtons inside a Row, in the bottomNavigationBar. Also, using the AppBar inside the WebviewScaffold.
Opening the second page replaces only the AppBar and the BottomNavigationBar, the WebView is still there.

Maybe I'm doing something wrong with the Navigator, I don't know. :/

```dart
MaterialApp(
title: 'Flutter Demo',
debugShowCheckedModeBanner: false,
theme: ThemeData(
primarySwatch: Colors.blue,
),
routes: {
"/": (_) => WebView(),
},
);

...

class _WebViewState extends State {

@override
Widget build(BuildContext context) {
return new WebviewScaffold(
url: "https://www.google.com",
appBar: AppBar(title: Text("First WebPage")),
bottomNavigationBar: bottomNavigation(context),
);
}

@override
void dispose() {
_flutterWebView.dispose();
super.dispose();
}

/// Buttons
Row bottomNavigation(context) {
return Row(
children: [
IconButton(
icon: Icon(Icons.list),
onPressed: () {
openList(context);
}),
IconButton(icon: Icon(Icons.phone_android), onPressed: openNativeScreen)
],
mainAxisAlignment: MainAxisAlignment.center,
);
}

///Actions
void openList(BuildContext context) {
Navigator.push(context, new NavigationRoute(builder: (context) => new ListScreen()));
}
}
```

Start by reproducing the sample using WebviewScaffold with its appBar, bottomNavigationBar, and Navigator.push call in openList. Inspect how the scaffold and pushed route are composed; done means the second page displays without the original WebView remaining underneath.

Scritto dal modello di indicizzazione a partire dal testo della issue.